U.S. patent application number 13/486065 was filed with the patent office on 2013-11-28 for techniques for forming a chalcogenide thin film using additive to a liquid-based chalcogenide precursor.
This patent application is currently assigned to International Business Machines Corporation. The applicant listed for this patent is David Brian Mitzi, Xiaofeng Qiu. Invention is credited to David Brian Mitzi, Xiaofeng Qiu.
Application Number | 20130312831 13/486065 |
Document ID | / |
Family ID | 49620641 |
Filed Date | 2013-11-28 |
United States Patent
Application |
20130312831 |
Kind Code |
A1 |
Mitzi; David Brian ; et
al. |
November 28, 2013 |
Techniques for Forming a Chalcogenide Thin Film Using Additive to a
Liquid-Based Chalcogenide Precursor
Abstract
Techniques for enhancing energy conversion efficiency in
chalcogenide-based photovoltaic devices by improved grain structure
and film morphology through addition of urea into a liquid-based
precursor are provided. In one aspect, a method of forming a
chalcogenide film includes the following steps. Metal chalcogenides
are contacted in a liquid medium to form a solution or a
dispersion, wherein the metal chalcogenides include a Cu
chalcogenide, an M1 and an M2 chalcogenide, and wherein M1 and M2
each include an element selected from the group consisting of: Ag,
Mn, Mg, Fe, Co, Cd, Ni, Cr, Zn, Sn, In, Ga, Al, and Ge. At least
one organic additive is contacted with the metal chalcogenides in
the liquid medium. The solution or the dispersion is deposited onto
a substrate to form a layer. The layer is annealed at a
temperature, pressure and for a duration sufficient to form the
chalcogenide film.
Inventors: |
Mitzi; David Brian;
(Mahopac, NY) ; Qiu; Xiaofeng; (Sleepy Hollow,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mitzi; David Brian
Qiu; Xiaofeng |
Mahopac
Sleepy Hollow |
NY
NY |
US
US |
|
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
49620641 |
Appl. No.: |
13/486065 |
Filed: |
June 1, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13479856 |
May 24, 2012 |
|
|
|
13486065 |
|
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|
Current U.S.
Class: |
136/264 ;
252/582; 257/613; 257/E21.114; 257/E29.068; 257/E31.027; 438/502;
438/95 |
Current CPC
Class: |
H01L 21/02628 20130101;
H01L 21/02568 20130101; H01L 31/032 20130101; H01L 21/02491
20130101; H01L 21/02422 20130101; H01L 21/02425 20130101; H01L
21/02557 20130101; H01L 21/0256 20130101 |
Class at
Publication: |
136/264 ;
257/613; 438/502; 438/95; 252/582; 257/E29.068; 257/E21.114;
257/E31.027 |
International
Class: |
G02B 5/24 20060101
G02B005/24; H01L 31/18 20060101 H01L031/18; H01L 21/208 20060101
H01L021/208; H01L 31/032 20060101 H01L031/032; H01L 29/12 20060101
H01L029/12 |
Claims
1. A composition, comprising: at least one organic additive and
metal chalcogenides in a liquid medium, wherein the at least one
organic additive is selected from the group consisting of urea,
thiourea and selenourea, wherein the metal chalcogenides comprise a
Cu chalcogenide, an M1 chalcogenide and an M2 chalcogenide, and
wherein M1 and M2 each comprise an element selected from the group
consisting of: Ag, Mn, Mg, Fe, Co, Cd, Ni, Cr, Zn, Sn, In, Ga, Al,
and Ge.
2. The composition of claim 1, wherein M1 is Sn.
3. The composition of claim 1, wherein M2 is Zn.
4. The composition of claim 1, wherein the Cu chalcogenide is
selected from the group consisting of: Cu.sub.2S, CuS, CuSe,
Cu.sub.2Se, Cu.sub.2SnS.sub.3, Cu.sub.2SnSe.sub.3,
Cu.sub.2Sn(S,Se).sub.3, Cu.sub.2ZnSnS.sub.4, Cu.sub.2ZnSnSe.sub.4,
Cu.sub.2ZnSn(S,Se).sub.4 and combinations comprising at least one
of the foregoing metal chalcogenides.
5. The composition of claim 1, wherein the M1 chalcogenide is
selected from the group consisting of: SnSe, SnS, SnSe.sub.2, SnS
.sub.2, Cu.sub.2SnS.sub.3, Cu.sub.2SnSe.sub.3,
Cu.sub.2Sn(S,Se).sub.3, Cu.sub.2ZnSnS.sub.4, Cu.sub.2ZnSnSe.sub.4,
Cu.sub.2ZnSn(S,Se).sub.4 and combinations comprising at least one
of the foregoing metal chalcogenides.
6. The composition of claim 1, wherein the M2 chalcogenide is
selected from the group consisting of: ZnS, ZnSe,
Cu.sub.2ZnSnS.sub.4, Cu.sub.2ZnSnSe.sub.4, Cu.sub.2ZnSn(S,Se).sub.4
and combinations comprising at least one of the foregoing metal
chalcogenides.
7. The composition of claim 1, wherein the at least one organic
additive is urea.
8. The composition of claim 1, further comprising: an M3
chalcogenide or an M3 salt in the liquid medium, wherein M3
comprises an element selected from the group consisting of: Na, K,
Li, Sb, Bi, Ca, Sr, Ba, and B.
9. The composition of claim 8, wherein the M3 chalcogenide or the
M3 salt is selected from the group consisting of: Sb.sub.2S.sub.3,
Sb.sub.2Se.sub.3, Sb.sub.2(S,Se).sub.3, Sb.sub.2S.sub.5, Na.sub.2S,
Na.sub.2Se, Na.sub.2(S,Se), K.sub.2S, K.sub.2Se, K.sub.2(S,Se),
Li.sub.2S, Li.sub.2Se, Li.sub.2(S,Se), Bi.sub.2S.sub.3,
Bi.sub.2Se.sub.3, Bi.sub.2(S,Se).sub.3, SbCl.sub.3, SbBr.sub.3,
SbI.sub.3, antimony(III) acetate, antimony(III) tartrate,
SbCl.sub.5, SbBr.sub.5, SbF.sub.3, SbF.sub.5, NaCl, NaBr, NaI, NaF,
NaOH, sodium acetate, Na.sub.2SO.sub.4, NaNO.sub.2,
Na.sub.2S.sub.2O.sub.3, NaNO.sub.3, Na.sub.2SO.sub.3,
Na.sub.2SeO.sub.3, KF, KCl, KBr, KI, KOH, potassium acetate,
K.sub.2SO.sub.4, K.sub.2S.sub.2O.sub.3 KNO.sub.2, KNO.sub.3,
K.sub.2SO.sub.3, K.sub.2S.sub.2O.sub.3, K.sub.2SeO.sub.3, LiF,
LiCl, LiBr, LiI, LiOH, lithium acetate, Li.sub.2SO.sub.4,
LiNO.sub.3, LiNO.sub.2, Li.sub.2SO.sub.3, Li.sub.2S.sub.2O.sub.3
Li.sub.2SeO.sub.3, BiF.sub.3, BiCl.sub.3, BiBr.sub.3, BiI.sub.3,
Bi(NO.sub.3).5H.sub.2O, bismuth(III) acetate, and bismuth(III)
citrate.
10. The composition of claim 1, wherein the liquid medium comprises
a solvent selected from the group consisting of: water, ammonium
hydroxide, ammonium hydroxide-water mixtures, ammonium
sulfide-ammonium hydroxide-water mixtures, alcohols, ethers,
glycols, aldehydes, ketones, alkanes, amines, dimethylsulfoxide
(DMSO), cyclic compounds, halogenated organic compounds and
combinations comprising at least one of the foregoing solvents.
11. A chalcogenide film formed by: contacting metal chalcogenides
in a liquid medium to form a solution or a dispersion, wherein the
metal chalcogenides comprise a Cu chalcogenide, an M1 chalcogenide
and an M2 chalcogenide, and wherein M1 and M2 each comprise an
element selected from the group consisting of: Ag, Mn, Mg, Fe, Co,
Cd, Ni, Cr, Zn, Sn, In, Ga, Al, and Ge; contacting at least one
organic additive with the metal chalcogenides in the liquid medium;
depositing the solution or the dispersion onto a substrate to form
a layer; and annealing the layer at a temperature, pressure and for
a duration sufficient to form the chalcogenide film.
12. The chalcogenide film of claim 11, wherein the chalcogenide
film has a formula:
Cu.sub.2-xM1.sub.1+yM2.sub.1+p(S.sub.1-zSe.sub.z).sub.4+q, (2)
wherein 0.ltoreq.x.ltoreq.1; -1.ltoreq.y.ltoreq.1;
-1.ltoreq.p.ltoreq.1; 0.ltoreq.z.ltoreq.1;
-1.ltoreq.q.ltoreq.1.
13. The chalcogenide film of claim 11, wherein the chalcogenide
film has a formula:
Cu.sub.2-xZn.sub.1+ySn.sub.1+p(S.sub.1-zSe.sub.z).sub.4+q, wherein
0.ltoreq.x.ltoreq.1; -1.ltoreq.y.ltoreq.1; -1.ltoreq.p.ltoreq.1;
0.ltoreq.z.ltoreq.1; and -1.ltoreq.q.ltoreq.1.
14. The chalcogenide film of claim 13, wherein x, y, z, p and q
are: 0.ltoreq.x.ltoreq.0.5; -0.5.ltoreq.y.ltoreq.0.5;
0.ltoreq.z.ltoreq.1; -0.5.ltoreq.p.ltoreq.0.5 and
-0.5.ltoreq.q.ltoreq.0.5, respectively.
15. A photovoltaic device, comprising: a substrate; a chalcogenide
film, which serves as an absorber layer, formed on the substrate
by: contacting metal chalcogenides in a liquid medium to form a
solution or a dispersion, wherein the metal chalcogenides comprise
a Cu chalcogenide, an M1 chalcogenide and an M2 chalcogenide, and
wherein M1 and M2 each comprise an element selected from the group
consisting of: Ag, Mn, Mg, Fe, Co, Cd, Ni, Cr, Zn, Sn, In, Ga, Al,
and Ge; contacting at least one organic additive with the metal
chalcogenides in the liquid medium; depositing the solution or the
dispersion onto a substrate to form a layer; and annealing the
layer at a temperature, pressure and for a duration sufficient to
form the chalcogenide film; an n-type semiconducting layer on the
chalcogenide film; and a top electrode on the n-type semiconducting
layer, wherein the photovoltaic device has a power conversion
efficiency of greater than or equal to about 8.1%.
16. The photovoltaic device of claim 15, wherein the substrate
comprises one or more of a metal foil substrate, aluminum foil
coated with a layer of molybdenum, a glass substrate with
conductive coating, a ceramic substrate with conductive coating and
a polymer substrate with a conductive coating.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation of U.S. application Ser.
No. 13/479,856 filed on May 24, 2012, the disclosure of which is
incorporated by reference herein.
FIELD OF THE INVENTION
[0002] The present invention relates to photovoltaic devices, such
as solar cells, and more particularly, to techniques for enhancing
energy conversion efficiency in chalcogenide-based photovoltaic
devices by improved grain structure and film morphology (e.g.,
crack and pinhole free) through addition of urea into a
liquid-based precursor.
BACKGROUND OF THE INVENTION
[0003] 100031 Copper quaternary chalcogenide compounds and alloys
are among the most promising absorber materials for photovoltaic
applications, due to their tunable and direct band gap, and very
high optical absorption coefficient in the visible and
near-infrared (IR) spectral range. Traditionally, these high
performance thin film photovoltaic compounds (such as copper indium
gallium selenide (CIGS)) are produced by vacuum-based thin film
deposition techniques, which require sophisticated equipment, high
processing temperatures (typically above 450 degrees Celsius
(.degree. C.)), and usually a post-deposition treatment in a
chalcogen (S or Se)-rich atmosphere (such as Se vapor or hydrogen
selenide/sulfide (H.sub.2Se/H.sub.2S)). Solution-based thin film
deposition techniques are regarded as a possible route to overcome
the cost and scalability issues faced by photovoltaic technology in
terms of competing with entrenched carbon-based electricity
production methods. In recent years, solution-processed copper zinc
tin sulfide (CZTS) or its selenide analogues have emerged as very
promising alternative photovoltaic absorber materials because of
not only using earth abundant and nontoxic elements, but also the
factor that the solution-processed CZTS devices are more efficient
than the vacuum-deposited devices. See, for example, T. Todorov, K.
Reuter, D. B. Mitzi, "High-Efficiency Solar Cell With
Earth-Abundant Liquid-Processed Absorber," Adv. Mater. 22,
E156-E159 (2010); S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, T. K.
Todorov, D. B. Mitzi, "Low band gap liquid-processed CZTSe solar
cell with 10.1% efficiency," Energy Environ. Sci., Issue 5, Feb.
2012, DOI: 10.1039/c2ee00056c; and B. Shin, O. Gunawan, Y. Zhu,
N.A. Bojarczuk, S. J. Chey, S. Guha, "Thin film solar cell with
8.4% power conversion efficiency using an earth-abundant
Cu.sub.2ZnSnS.sub.4 absorber," Prog. Photovolt: Res. Appl. (2011)
DOI: 10.1002/pip.
[0004] This family of absorbers, also referred to as kesterites,
consists of Cu.sub.2ZnSnS.sub.4 (CZTS), as well as
Cu.sub.2ZnSnSe.sub.4 (CZTSe) and more generally
Cu.sub.2ZnSn(S,Se).sub.4 (CZTSSe), with the S:Se ratio governing
the band gap in the material. Besides tailoring the band gap using
the S:Se ratio, substitution of Ge for Sn (i.e.,
Cu.sub.2Zn(Sn,Ge)(S,Se).sub.4) can also be employed. The above
family of materials will be generally referred to as
CZTS-based.
[0005] A challenge faced by solution-based deposition methods is
the difficulty in controlling the grain structure and film
morphology of the absorber layer. Small grain size and poor film
morphology severely limit solar cell efficiency. Namely, grain
boundaries can act as recombination centers for the photogenerated
electrons and holes, which is detrimental to the device
performance. In general, grain sizes on the order of absorber layer
thickness (micrometer (.mu.m)-length scale) are desirable in order
to minimize such recombination effects. Film cracks and/or pinholes
are another problem limiting the quality of the absorber layer, as
cracks and pinholes can lead to device shunting. Therefore,
approaches that result in good grain structure and crack- and
pinhole-free films would be desirable.
[0006] So far the best CZTS-based photovoltaic devices are prepared
by a hydrazine-based solution technique. See, for example, T.
Todorov et al., "High-Efficiency Solar Cell with Earth-Abundant
Liquid-Processed Absorber," Adv. Mater. 22, E156-E159 (2010). With
this approach, over 10% energy conversion efficiency has been
achieved. See, for example, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu,
T. K. Todorov, D. B. Mitzi, "Low band gap liquid-processed CZTSe
solar cell with 10.1% efficiency," Energy Environ. Sci., Issue 5,
February 2012, DOI: 10.1039/c2ee00056c; D. A. R. Barkhouse, O.
Gunawan, T. Gokmen, T. K. Todorov, D. B. Mitzi, "Device
characteristics of a 10.1% hydrazine-processed
Cu.sub.2ZnSn(Se,S).sub.4 solar cell," Prog. Photovolt: Res. Appl.
20, 6-11 (January 2012). However, hydrazine is an explosive and
highly toxic solvent, which must be used under carefully controlled
conditions (generally in an inert atmosphere such as nitrogen or
argon). Therefore, there is a need to develop approaches that do
not employ hydrazine, but still enable the deposition of
high-quality films.
[0007] An alternative hydrazine-free nanoparticle-based method
yielded a CZTS photovoltaic device with 7.2% efficiency using
organic amines. See Q. Guo, G. M. Ford, W. Yang, B. C. Walker, E.
A. Stach, H. W. Hillhouse, R. Agrawal, "Fabrication of 7.2%
Efficient CZTSSe Solar Cells Using CZTS Nanocrystals," J. Am. Chem.
Soc., 2010, 132, 17384-17386. Although this method avoided using
highly toxic and explosive hydrazine, it involves heavy use of
toxic and expensive organic reagents and an anneal in toxic
selenium vapor, which therefore does not necessarily eliminate the
safety and environmental problems, but may also introduce unwanted
carbon impurities and negatively impact the device performance. The
same group also reported the preparation of a CZTS photovoltaic
device using less expensive and less toxic dimethyl sulfoxide
(DMSO) as solvent. This method yielded an energy conversion
efficiency of 4.1%, which may be limited by the small grains (on
the order of a couple of hundred nanometers or smaller). See W. Ki,
H. W. Hillhouse, "Earth-Abundant Element Photovoltaics Directly
from Soluble Precursors with High Yield Using a Non-Toxic Solvent,"
Adv. Energy Mater., 2011, 1, 732-735.
[0008] Patent Application Number W02011/066205A1, filed by L. K.
Johnson et al. entitled "Aqueous process for producing crystalline
copper chalcogenide nanoparticles, the nanoparticles so-produced,
and inks and coated substrates incorporating the nanoparticles"
introduced the synthesis of ink in an aqueous medium and developed
kesterite CZTS thin films. Although, this method provided the
possible route to make crystalline CZTS nanoparticles and films
developed from such nanoparticles, it was not demonstrated to be
useful in the preparation of high performance CZTS devices. On the
other hand, the ligands and organic additives described therein may
lead to unwanted carbon contamination in the films, which could
impact the grain structures and film morphology, therefore possibly
affecting the photovoltaic efficiency.
[0009] U.S. Patent Application Publication Number 2011/0097496 A2
filed by Mitzi et al., entitled "Aqueous-Based Method of Forming
Semiconductor Film and Photovoltaic Device Including the Film"
provides an aqueous-based non-hydrazine approach to prepare CZTS
thin films and photovoltaic devices. However, it has been found
that the CZTS films prepared by this method without any hydrazine
exhibit small grains (a couple of hundred nanometers) and surface
cracking. The best devices fabricated from these films reached
efficiency of around 7%.
[0010] The above-described approaches generally either employ
hydrazine or, for water-based approaches, generally have issues
with reproducibly being able to produce CZTS films with good
morphology and grain size, particularly for pure sulfide CZTS
films. Therefore, a method of improving the grain structure and
film morphology of CZTS/CZTSe-based absorber layer prepared from
non-toxic solution-based techniques, preferrably an aqueous
solution, would be desirable.
SUMMARY OF THE INVENTION
[0011] The present invention provides techniques for enhancing
energy conversion efficiency in chalcogenide-based photovoltaic
devices by improving grain structure and film morphology through
addition of urea into a liquid-based precursor. In one aspect of
the invention, a method of forming a chalcogenide film is provided.
The method includes the following steps. Metal chalcogenides are
contacted in a liquid medium to form a solution or a dispersion,
wherein the metal chalcogenides include a Cu chalcogenide, an M1
chalcogenide and an M2 chalcogenide, and wherein M1 and M2 each
include an element selected from the group including: Ag, Mn, Mg,
Fe, Co, Cd, Ni, Cr, Zn, Sn, In, Ga, Al, and Ge. Optionally, an
additional M3 chalcogenide or M3 salt is contacted with the metal
chalcogenide, wherein M3 includes an element selected from the
group including: Na, K, Li, Sb, Bi, Ca, Sr, Ba, and B. At least one
organic additive is contacted with the metal chalcogenides in the
liquid medium. The solution or the dispersion is deposited onto a
substrate to form a layer. The layer is annealed at a temperature,
pressure and for a duration sufficient to form the chalcogenide
film.
[0012] In another aspect of the invention, a composition is
provided. The composition includes at least one organic additive
and metal chalcogenides in a liquid medium, wherein the metal
chalcogenides include a Cu chalcogenide, an M1 chalcogenide and an
M2 chalcogenide, and wherein M1 and M2 each include an element
selected from the group including: Ag, Mn, Mg, Fe, Co, Cd, Ni, Cr,
Zn, Sn, In, Ga, Al, and Ge. Optionally, the composition includes an
additional M3 chalcogenide or M3 salt, wherein M3 includes an
element selected from the group including: Na, K, Li, Sb, Bi, Ca,
Sr, Ba, and B.
[0013] In another aspect of the invention, a photovoltaic device is
provided. The photovoltaic device includes a substrate; a
chalcogenide film formed on the substrate by the above-described
method, which serves as an absorber layer; an n-type semiconducting
layer on the chalcogenide film; and a top electrode on the n-type
semiconducting layer. The photovoltaic device has a power
conversion efficiency of greater than or equal to about 8.1%.
[0014] A more complete understanding of the present invention, as
well as further features and advantages of the present invention,
will be obtained by reference to the following detailed description
and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a diagram illustrating an exemplary methodology
for fabricating a metal chalcogenide film from additive-containing
pure solution or particle-based routes according to an embodiment
of the present invention;
[0016] FIG. 2 is a cross-sectional diagram illustrating a starting
structure for fabricating a photovoltaic device, e.g., a substrate
formed from a conductive material or a substrate coated with a
layer of conductive material according to an embodiment of the
present invention;
[0017] FIG. 3 is a cross-sectional diagram illustrating a
chalcogenide film absorber layer having been formed on the
substrate according to an embodiment of the present invention;
[0018] FIG. 4 is a cross-sectional diagram illustrating an n-type
semiconducting layer having been formed on the chalcogenide film
and a top electrode having been formed on the n-type semiconducting
layer according to an embodiment of the present invention;
[0019] FIG. 5A is a scanning electron micrograph (SEM) image of a
top view of a sample metal chalcogenide film prepared from ink
containing no urea but some ammonium sulfide according to an
embodiment of the present invention;
[0020] FIG. 5B is an SEM image of a top view of a sample metal
chalcogenide film prepared from ink containing 0.2M urea and 0.5
atomic percent (at. %) NaF according to an embodiment of the
present invention;
[0021] FIG. 5C is an SEM image of a cross-sectional view of the
film of FIG. 5A according to an embodiment of the present
invention;
[0022] FIG. 5D is a SEM image of a cross-sectional view of the film
of FIG. 5B according to an embodiment of the present invention;
[0023] FIG. 6A is a SEM image of a top view of a sample metal
chalcogenide film prepared from ink using only Na as additive
according to an embodiment of the present invention;
[0024] FIG. 6B is a SEM image of a cross-sectional view of the film
of FIG. 6A according to an embodiment of the present invention;
[0025] FIG. 6C is a SEM image of a top view of a sample metal
chalcogenide film prepared from ink using only urea as additive
according to an embodiment of the present invention;
[0026] FIG. 6D is a SEM image of a cross-sectional view of the film
of FIG. 6C according to an embodiment of the present invention;
[0027] FIG. 6E is a SEM image of a sample metal chalcogenide film
prepared from ink using both urea and Na as additives according to
an embodiment of the present invention;
[0028] FIG. 6F is a SEM image of a cross-sectional view of the film
of FIG. 6E according to an embodiment of the present invention;
[0029] FIG. 7A is a graph illustrating electrical characteristics
of a photovoltaic device based on a metal chalcogenide film
prepared using only Na as an additive according to an embodiment of
the present invention;
[0030] FIG. 7B is a graph illustrating electrical characteristics
of a photovoltaic device based on a metal chalcogenide film
prepared using only urea as an additive according to an embodiment
of the present invention;
[0031] FIG. 7C is a graph illustrating electrical characteristics
of a photovoltaic device based on a metal chalcogenide film
prepared using both Na and urea as additives according to an
embodiment of the present invention;
[0032] FIG. 8A is a SEM image of a cross-sectional view of a sample
metal chalcogenide film prepared using the present techniques
according to an embodiment of the present invention;
[0033] FIG. 8B is a SEM image of a top view of the film of FIG. 8A
according to an embodiment of the present invention;
[0034] FIG. 9 is a graph illustrating electrical characteristics of
a photovoltaic device based on the film of FIGS. 8A and 8B
according to an embodiment of the present invention;
[0035] FIG. 10A is a SEM image of a top view of another sample
metal chalcogenide film prepared using the present techniques
according to an embodiment of the present invention;
[0036] FIG. 10B is a SEM image of a cross-sectional view of the
film of FIG. 10A according to an embodiment of the present
invention;
[0037] FIG. 11 is a graph illustrating powder X-ray diffraction
patterns of the film of FIGS. 10A and 10B according to an
embodiment of the present invention;
[0038] FIG. 12 is a graph illustrating electrical characteristics
of a photovoltaic device based on the film of FIGS. 10A and 10B
according to an embodiment of the present invention;
[0039] FIG. 13A is a SEM image of a top view of yet another sample
metal chalcogenide film prepared using the present techniques
according to an embodiment of the present invention;
[0040] FIG. 13B is a SEM image of a cross-sectional view of the
film of FIG. 13A according to an embodiment of the present
invention; and
[0041] FIG. 14 is a graph illustrating electrical characteristics
of a photovoltaic device based on the film of FIGS. 13A and 13B
according to an embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0042] For clarity of description, definitions of some terms used
throughout the description are now provided:
[0043] The term "ink," as used herein refers to a liquid composed
of at least one solvent, at least one kind of metal chalcogenide
solid particle and at least one organic additive. The solvent can
be water or nonaqueous solvent and accounts for from about 1% to
about 99% of a weight of the ink. The solid metal chalcogenide
particles account for from about 0.01% to about 50% of the weight
of the ink. The shape of the solid metal chalcogenide particles can
be, but is not limited to, spheres, cubes, rods, flakes and stars.
The size of the solid metal chalcogenide particles (measured, for
example, as a longest lateral dimension, e.g., longest width,
longest length, etc.) can be, but is not limited to from about 5
nanometers (nm) to about 1,000 nm, for example, from about 5 nm to
about 200 nm. The organic additive accounts for from about 0.001%
to about 50% of the weight of the ink. This ink can be used to form
a metal chalcogenide film. The ink may also be referred to herein
as a "suspension," "dispersion" or "particle-based solution," and
these terms will be used synonymously herein. The term "ink" also
encompasses a liquid composed of at least one solvent, at least one
dissolved metal salt, at least one dissolved source of chalcogenide
and at least one organic additive. In this case, the ink can be
considered a "pure solution" since there are no dispersed particles
and everything in the ink is fully dissolved. Thus, the term "ink,"
as used herein encompasses either a solution or dispersion of metal
chalcogenides and organic additive(s) in a liquid medium.
[0044] The family of absorbers referred to as "kesterites" consists
of Cu.sub.2ZnSnS.sub.4 (CZTS), as well as Cu.sub.2ZnSnSe.sub.4
(CZTSe) and more generally Cu.sub.2ZnSn(S,Se).sub.4 (CZTSSe), with
the S:Se ratio governing the band gap in the material. Besides
tailoring the band gap using the S:Se ratio, substitution of Ge for
Sn (i.e., Cu.sub.2Zn(Sn,Ge)(S,Se).sub.4) can also be employed. The
above formulas for kesterites represent the ideal stoichiometries.
As described above, for photovoltaic applications, it is found that
non-stoichiometric compositions yield higher conversion efficiency,
with generally a copper poor and zinc rich composition yielding the
highest efficiencies. When the term kesterite or CZTS is employed
in the present description it is meant to refer to the full range
of kesterite compositions based on Cu, Zn, Sn, Ge, Sn, S, Se, as
well as including other common impurity atoms such as Na, K, Sb,
Bi, Li. The term "CIGS," as used herein refers to a material with
the chalcopyrite structure of the formula CuInS.sub.2,
CuInSe.sub.2, Cu(Ga,In)Se.sub.2, CuIn(S,Se).sub.2,
Cu(Ga,In)(S,Se).sub.2 and may also include other impurity atoms
such as Na, K, Sb, Bi, Li, Ca, Sr, Ba and B.
[0045] The term "chalcogenides," as used herein, refers to
compounds that contain chalcogens such as S, Se and/or Te. In one
exemplary embodiment, the chalcogens used in accordance with the
present techniques are S and/or Se.
[0046] The present techniques relate to adding additives into a
metal chalcogenide-containing liquid medium to improve grain
structure and morphology of copper-based quaternary chalcogenide
thin films prepared from such liquid, which leads to the
enhancement of photovoltaic conversion efficiency of the devices
developed from the films.
[0047] FIG. 1 is a diagram illustrating an exemplary methodology
100 for fabricating a chalcogenide film from additive-containing
pure solution and particle-based routes. To begin the process, a
precursor composition is prepared. The term "precursor" refers to
the fact that the composition contains the elements needed to form
the final film. However, until the composition is deposited and
annealed (as described below) to enable formation of the desired
crystal structure, the composition is only a precursor to the final
film. As will be described in detail below, the precursor
composition will be deposited onto a substrate, which after an
annealing process will form the chalcogenide film. The precursor
composition can be either a solution or a dispersion (i.e., a
particle-based solution) containing dissolved components and/or
solid particles, and as provided above is also referred to herein
as an "ink." Ideally the target during the precursor composition
(ink) formation is a true solution with all of the precursors
completely dissolved in a liquid medium, which will facilitate film
deposition. However in practice, it is often the case that some or
all of the metal chalcogenide precursors are not able to dissolve
into any solvents. Thus, an alternative is to use a
suspension/dispersion as an ink that contains all of the precursors
(i.e., a particle-based ink).
[0048] Namely, in step 102, metal chalcogenides are contacted
(i.e., mixed) in a liquid medium to form a solution or a dispersion
(also referred to herein as a "metal chalcogenide-containing liquid
medium"). According to an exemplary embodiment, the metal
chalcogenides include a copper (Cu) chalcogenide, a first metal
(M1) chalcogenide and a second (M2) chalcogenide. M1 and M2 each
include an element selected from the group including silver (Ag),
manganese (Mn), magnesium (Mg), iron (Fe), cobalt (Co), cadmium
(Cd), nickel (Ni), chromium (Cr), zinc (Zn), tin (Sn), indium (In),
gallium (Ga), aluminum (Al), and germanium (Ge). According to an
exemplary embodiment, M1 is Sn and M2 is Zn.
[0049] Optionally, in step 104, an additional M3 chalcogenide or M3
salt is contacted with the liquid medium, wherein M3 includes an
element selected from the group including, sodium (Na), potassium
(K), lithium (Li), antimony (Sb), bismuth (Bi), calcium (Ca),
strontium (Sr), barium (Ba), and boron (B).
[0050] Suitable Cu chalcogenides include, but are not limited to,
Cu.sub.2S, CuS, CuSe, Cu.sub.2Se, Cu.sub.2SnS.sub.3,
Cu.sub.2SnSe.sub.3, Cu.sub.2Sn(S,Se).sub.3, Cu.sub.2ZnSnS.sub.4,
Cu.sub.2ZnSnSe.sub.4, Cu.sub.2ZnSn(S,Se).sub.4 and combinations
including at least one of the foregoing metal chalcogenides.
Suitable M1 chalcogenides include, but are not limited to, SnSe,
SnS, SnSe.sub.2, SnS.sub.2, Cu.sub.2SnS.sub.3, Cu.sub.2SnSe.sub.3,
Cu.sub.2Sn(S,Se).sub.3, Cu.sub.2ZnSnS.sub.4, Cu.sub.2ZnSnSe.sub.4,
Cu.sub.2ZnSn(S,Se).sub.4 and combinations including at least one of
the foregoing metal chalcogenides. Suitable M2 chalcogenides
include, but are not limited to, ZnS, ZnSe, Cu.sub.2ZnSnS.sub.4,
Cu.sub.2ZnSnSe.sub.4, Cu.sub.2ZnSn(S,Se).sub.4 and combinations
including at least one of the foregoing metal chalcogenides.
[0051] Suitable M3 chalcogenides or M3 salts include but are not
limited to Sb.sub.2S.sub.3, Sb.sub.2Se.sub.3, Sb.sub.2(S,Se).sub.3,
Sb.sub.2S.sub.5, Na.sub.2S, Na.sub.2Se, Na.sub.2(S,Se), K.sub.2S,
K.sub.2Se, K.sub.2(S,Se), Li.sub.2S, Li.sub.2Se, Li.sub.2(S,Se),
Bi.sub.2S.sub.3, Bi.sub.2Se.sub.3, Bi.sub.2(S,Se).sub.3,
SbCl.sub.3, SbBr.sub.3, SbI.sub.3, antimony(III) acetate,
antimony(III) tartrate, SbCl.sub.5, SbBr.sub.5, SbF.sub.3,
SbF.sub.5, NaC1, NaBr, Nal, NaF, NaOH, sodium acetate,
Na.sub.2SO.sub.4, NaNO.sub.2, NaNO.sub.3, Na.sub.2SO.sub.3,
Na.sub.2SeO.sub.3, Na.sub.2S.sub.2O.sub.3, KF, KCl, KBr, KI, KOH,
potassium acetate, K.sub.2SO.sub.4, KNO.sub.2, KNO.sub.3,
K.sub.2SO.sub.3, K.sub.2S.sub.2O.sub.3 K.sub.2SeO.sub.3, LiF, LiCl,
LiBr, LiI, LiOH, lithium acetate, Li.sub.2SO.sub.4, LiNO.sub.3,
LiNO.sub.2, Li.sub.2SO.sub.3, Li.sub.2S.sub.2O.sub.3,
Li.sub.2SeO.sub.3, BiF.sub.3, BiCl.sub.3, BiBr.sub.3, BiI.sub.3,
Bi(NO.sub.3).5H.sub.2O, bismuth(III) acetate, and bismuth(III)
citrate.
[0052] According to an exemplary embodiment, the liquid medium is a
solvent such as water or a non-aqueous liquid, the latter being
either an organic or inorganic liquid. Preferably, the liquid
medium is a solvent that can be substantially eliminated (e.g.,
greater than 90% of the solvent can be removed) by evaporation at a
temperature lower than the decomposition temperature for the
solvent. Suitable exemplary solvents that meet this criterion are
provided below. For example, water (an inorganic solvent) can be
evaporated at temperature of about 100.degree. C. and ethanol (an
organic solvent) evaporates at a temperature of greater than about
78.degree. C. Suitable solvents include, but are not limited to,
water, ammonium hydroxide, ammonium hydroxide-water mixtures,
ammonium sulfide-ammonium hydroxide-water-mixtures, alcohols,
ethers, glycols, aldehydes, ketones, alkanes, amines,
dimethylsulfoxide (DMSO), cyclic compounds, halogenated organic
compounds and combinations including at least one of the foregoing
solvents.
[0053] The M3 chalcogenide or metal salt which is optionally added
in step 104 may be added to the metal chalcogenide-containing
liquid medium to improve the film formation and/or affect certain
properties of the film. Suitable M3 metals were provided above.
These M3 metals become incorporated into the metal
chalcogenide-containing liquid. A small amount (e.g., from about
0.0001 percent by weight (% wt) to about 10% wt) of these metals
may be added into the metal chalcogenide-containing liquid medium
to improve the film formation or certain physical properties. For
example, Na is a well known additive in photovoltaic films that is
used to change the conductivity of the material. See, for example,
A. Rockett, "The effect of Na in polycrystalline and epitaxial
single-crystal CuIn.sub.(1-x)Ga.sub.(x)Se.sub.2," Thin Solid Films,
480-481, 2 (2005); H. Nukala, et al. "Synthesis of optimized CZTS
thin films for photovoltaic absorber layers by sputtering from
sulfide targets and sulfurization" Mater. Res. Soc. Symp. Proc.
1268-EE03-04 (2010), the contents of each of which are incorporated
by reference herein.
[0054] The term "improved grain size," as used herein, refers to
targeting grain sizes on the order of the absorber layer thickness
(micrometer (m)-length scale), which is desirable in order to
minimize the photogenerated electron and hole recombination at the
grain interfaces. Preferably, average gain size is from about 300
nm to about 100 .mu.m. For example, average grain size is from
about 500 nm to about 10 .mu.m. For example, FIG. 5D (described
below) shows the typical good grain size in the film made from
urea-containing ink is on the order of the film thickness (about 1
.mu.m)
[0055] The term "improved film morphology" described herein refers
to the film with less or free of cracks and pin holes. By way of
example only, films prepared using the present techniques if not
completely free of cracks and/or pinholes will have cracks with a
length that is less than 5 .mu.m and a width that is less than 1
.mu.m, e.g., a length less than 1 .mu.m and a width less than 500
nm, and pinholes having a diameter of less than 1 .mu.m, for
example, a diameter of less than 500 nm. Pinhole means a void that
goes all the way from a top of the film to the back contact. For
example, FIG. 6A (described below) shows the cracks in CZTS
prepared without urea. Most of the cracks are longer than 10 .mu.m
and wider than 2 .mu.m. FIG. 6E (described below) shows the cracks
and pinholes in the film prepared from ink with urea and Na
addition. The crack is shorter than 3 .mu.m and narrower than 300
nm. The pinholes are smaller than 200 nm in diameter.
[0056] Next, in step 106, an organic additive(s) is/are contacted
(mixed) with the metal chalcogenides in the liquid medium.
According to an exemplary embodiment, the organic additive is a
molecule of a form:
R1=CR2R3, (1)
wherein R1 is an element selected from group 16 of the periodic
table of elements (i.e., oxygen (O), sulfur (S), selenium (Se), and
tellurium (Te), C is carbon, and R2 and R3 each represent any
element or functional group. R2 and R3 can be the same or different
element/functional group. According to an exemplary embodiment, R2
and R3 are each primary amine groups.
[0057] By way of example only, suitable organic additives in
accordance with Equation 1 include, but are not limited to, urea,
thiourea and selenourea. Urea is preferred due to its abundance,
low cost and non-toxicity. Urea can easily decompose to NH.sub.3
and CO.sub.2 in the presence of water at temperatures below
150.degree. C. Urea is also very soluble in water (107.9 g/100 mL
20.degree. C.) and many other solvents like alcohols, and therefore
can be easily introduced into many solution-based processes. The
present techniques are not limited to the use of a single organic
additive. For instance, the solubility of urea in ethanol (50 g/L)
is limited. Therefore another possible additive, such as thiourea,
(35 g/L) (in addition to urea) is added to the liquid medium to
reach the above-stated concentration (e.g., a combined
concentration of greater than 70 g/L) of organic additive and thus
achieve adequate grain growth in the film.
[0058] The organic additive(s) can be introduced into the metal
chalcogenide-containing liquid medium (from step 102) in several
different ways. For instance, the organic additive(s) can be first
dissolved in a liquid medium to form an organic additive-containing
liquid medium. The liquid medium can be a solvent. Suitable
solvents were provided above. The organic additive-containing
liquid medium can then be mixed with the chalcogenide-containing
liquid medium under agitation, stirring and/or sonication.
Alternatively, the solid state organic additive(s) can be added
directly to the chalcogenide-containing liquid medium also under
agitation, stirring and/or sonication.
[0059] Accordingly, the organic additive(s) should sufficiently
dissolve in the liquid medium. Preferably, the solubility of the
organic additive(s) in the liquid medium is from about 1 micromolar
(.mu.M) to about 100 molar (M), e.g., the solubility is from about
1 millimolar (mM) to about 10 M.
[0060] One characteristic of the additive used in this technique is
that it is easy to be removed from the film materials upon gentle
heating. Generally, it is thought that it is preferable to avoid
the introduction of organic additives to solutions and slurries
used for the deposition of metal chalcogenide films, because the
additives are thought to leave residue of carbon or oxygen that can
lead to inferior device performance. The organic additive(s) in the
present techniques are therefore designed to be readily removed
from the metal chalcogenides upon heat treatment (step 110) after
solution deposition (step 108). The additives of choice are
targeted to be chemicals that can decompose or evaporate upon
gentle heat treatment, for example, at temperatures lower than
about 300 degrees Celsius (.degree. C.), more preferably at a
temperature of from about 30.degree. C. to about 150.degree. C.
[0061] Also, in order to facilitate removal of the organic
additive(s) upon annealing, the organic additive(s) is preferably
added after the metal chalcogen bonding has formed (either particle
or ionic species) to avoid strong coordination between metal ions
and additives. Namely, step 102 serves to mix/bond the metal
chalcogenides within the liquid medium. Adding the organic
additive(s) in step 106, after this metal chalcogen bonding takes
place, will help ensure that the organic additive(s) are weakly or
moderately attached to the surface of the metal chalcogenide
particles, particle agglomerates or metal compounds (for example,
the binding energy is less than 150 kJ/mol, e.g., the binding
energy is less than 50 kJ/mol); therefore the organic additive(s)
can be removed without leaving chemical residues upon gentle
heat-treatment, preferably at temperatures lower than 300.degree.
C., more preferably, from about 30.degree. C. to about 150.degree.
C.
[0062] The precursor composition now formed may be used in the
fabrication of a chalcogenide film as described in detail below. As
provided above, the precursor composition can be a solution, or a
dispersion (the precursor composition solution or dispersion also
referred to herein as an ink). Accordingly, based on the above
description, the precursor composition will contain at least one
organic additive and metal chalcogenides in a liquid medium. The
metal chalcogenides include 1) a Cu chalcogenide, 2) an M1
chalcogenide and 3) an M2 chalcogenide. M1 and M2 each include an
element selected from: Ag, Mn, Mg, Fe, Co, Cd, Ni, Cr, Zn, Sn, In,
Ga, Al, and Ge. Further as provided above, optionally, an
additional M3 chalcogenide or M3 salt is contacted with the liquid
medium, wherein M3 is an element selected from the group including:
Na, K, Li, Sb, Bi, Ca, Sr, Ba, and B.
[0063] According to an exemplary embodiment, a concentration of the
metal chalcogenide species in the precursor composition is from
about 1 .mu.M to about 100M, e.g., from about 10 .mu.M to about 1M.
In this example, the fluid medium accounts for from about 10 weight
percent (wt %) to about 99 wt % of the precursor composition.
Further, in this example, a concentration of the organic
additive(s) in the precursor composition varies from about 1
micromolar to the solubility limit of the additive in certain
solvents at a given temperature. For example, the upper limit of
the urea concentration in water at 20.degree. C. is 17.84 M.
According to an exemplary embodiment, the concentration of the
organic additive(s) in the precursor composition is from about 1
.mu.M to about 100M, e.g., from about 10 .mu.M to about 10M.
[0064] The process for using the precursor composition to form a
chalcogenide film is now described. In step 108, the precursor
composition (i.e., solution or dispersion/ink) is deposited onto a
substrate to form a layer. By way of example only, suitable
substrates include, but are not limited to, a metal foil substrate,
aluminum foil coated with a layer of molybdenum, a glass substrate
with conductive coating, a ceramic substrate with conductive
coating and/or a polymer substrate with a conductive coating. The
present techniques may be employed to form an absorber layer of a
photovoltaic device (see below). The conductive coating/layer or
substrate can, in that instance, serve as an electrode of the
device. In one embodiment the substrate is metal or alloy foil
containing as non-limiting examples molybdenum, aluminum, titanium,
iron, copper, tungsten, steel or combinations thereof. In another
embodiment the metal or alloy foil is coated with an ion diffusion
barrier and/or an insulating layer succeeded by a conductive layer.
In another embodiment the substrate is polymeric foil with a
metallic or other conductive layer (e.g., transparent conductive
oxide, carbon) deposited on the top of it. In one preferred
embodiment, regardless of the nature of the underlying substrate
material or materials, the surface contacting the liquid layer
contains molybdenum.
[0065] Suitable processes for depositing the precursor composition
onto the substrate include, but are not limited to spin-coating,
dip-coating, doctor blading, curtain coating, slide coating,
spraying, slit casting, meniscus coating, screen printing, ink jet
printing, pad printing, flexographic printing and gravure printing.
After a liquid layer of the precursor composition is deposited on
the surface of the substrate, the process of drying the film and
removing some part of the excess chalcogen may be initiated by
evaporation, by means of exposure to ambient or controlled
atmosphere or vacuum that may be accompanied with a thermal
treatment, referred to as preliminary anneal, to fabricate a
substrate coated with a hybrid precursor including discrete
particles and surrounding media. This surrounding media is formed
by solidification of the dissolved component. The process of
depositing the precursor composition onto the substrate and of
drying the film and removing some part of the excess chalcogen may
be repeated multiple times to increase film thickness (i.e., to
achieve a desired thickness) before proceeding to step 110.
[0066] Next, in step 110, the layer (deposited in step 108) is
annealed (also referred to as a heat treatment) at a temperature,
pressure and for a duration sufficient to form the chalcogenide
film. Namely, the metal chalcogenide precursor layer is heated to a
temperature sufficient to induce reaction/recrystallization and
grain growth among the metal chalcogenide species therein to form a
nominally single-phase film with an average grain size with at
least one dimension greater than 50 nm, e.g., greater than 200 nm,
with the desired composition.
[0067] According to an exemplary embodiment, the heat treatment
involves heating the film to a temperature of from about
200.degree. C. to about 800.degree. C., for example, from about
300.degree. C. to about 700.degree. C., e.g., from about
450.degree. C. to about 650.degree. C., at a pressure of from about
1 .mu.Pa(scal) to about 1>10.sup.6 Pa(scal), for a duration of
from about 10 seconds to about 120 minutes, e.g., from about 2
minutes to about 60 minutes. The step of heat treating is
preferably carried out in an atmosphere including at least one of
nitrogen, argon, helium, forming gas, and a mixture containing at
least one of the foregoing gases. This atmosphere can further
include vapors of at least one of S, Se, Sn and a compound
containing S, Se and/or Sn (e.g., H.sub.2S, H.sub.2Se, SnS, SnSe,
SnS.sub.2 or SnSe.sub.2). The ratio of S and Se sources in the
vapor can be selected to impact the final S:Se ratio in the final
film. The film produced in this manner preferably contains at least
80% by mass of the targeted compound, more preferably at least 90%
by mass of the targeted compound and even more preferably at least
95% by mass of the targeted compound. The targeted compound is, for
example, the CZTS, CZTSe or CZTSSe kesterite compound of the
formula provided above.
[0068] The anneal can be carried out by any technique known to one
of skill in the art, including but not limited to, furnace, hot
plate, infrared or visible radiation and convective (e.g., laser,
lamp furnace, rapid thermal anneal unit, resistive heating of the
substrate, heated gas stream, flame burner, electric arc and plasma
jet). The intimate contact between the two components of the hybrid
precursor (particle component and solidified dissolved component)
for most embodiments enables limiting the anneal duration to less
than 60 minutes (as provided above).
[0069] Other techniques for fabricating kesterite films are
described in U.S. patent application Ser. No. 13/207,269, filed by
Bag et al., entitled "Capping Layers for Improved Crystallization,"
and in U.S. patent application Ser. No. 13/207,187, filed by Mitzi
et al., entitled "Particle-Based Precursor Formation Method and
Photovoltaic Device Thereof," and in U.S. patent application Ser.
No. 13/207,248, filed by Mitzi et al., entitled "Process for
Preparation of Elemental Chalcogen Solutions and Method of
Employing Said Solutions in Preparation of Kesterite films,"
(hereinafter "U.S. patent application Ser. No. 13/207,248") and in
U.S. Patent Application Publication Number 2011/0097496, filed by
Mitzi et al., entitled "Aqueous-Based Method of Forming
Semiconductor Film and Photovoltaic Device Including the Film," the
entire contents of each of which are incorporated by reference
herein.
[0070] The result is a chalcogenide film having been formed on the
substrate. The obtained film on the substrate may then be used for
the desired application, such as, optical, electrical,
anti-friction, bactericidal, catalytic, photo-catalytic,
electromagnetic shielding, wear-resistance, and diffusion barrier.
As will be described in detail below, in one exemplary
implementation, the above-described process is used to fabricate
the absorber layer of a photovoltaic device, i.e., wherein the
chalcogenide film serves as the absorber layer.
[0071] In one exemplary embodiment, the chalcogenide film formed
has a formula:
Cu.sub.2-xM1.sub.1+yM2.sub.1+p(S.sub.1-zSe.sub.z).sub.4+q, (2)
wherein 0.ltoreq.x<1; -1.ltoreq.y.ltoreq.1;
-1.ltoreq.p.ltoreq.1; 0.ltoreq.z.ltoreq.1; -1.ltoreq.q.ltoreq.1.
Thus, the present techniques can be used to fabricate both CIGS
(chalcopyrite) and CZTS (kesterite) chalcogenide films. For
kesterite materials additives and non-stoichiometry are often
desired. For example, in one exemplary embodiment, M1 and M2 are Zn
and Sn, respectively, and the chalcogenide film formed has a
formula:
Cu.sub.2-xZn.sub.1+ySn.sub.1+p(S.sub.1-zSe.sub.z).sub.4+q, (3)
wherein 0.ltoreq.x.ltoreq.1; -1.ltoreq.y.ltoreq.1;
-1.ltoreq.p.ltoreq.1; 0.ltoreq.z.ltoreq.1; and
-1.ltoreq.q.ltoreq.1, for example, wherein x, y, z, p and q are:
0.ltoreq.x.ltoreq.0.5; -0.5.ltoreq.y.ltoreq.0.5; -0.5p.ltoreq.0.5;
0.ltoreq.z.ltoreq.1; and -0.5.ltoreq.q.ltoreq.0.5,
respectively.
[0072] The implementation of the present techniques for the
fabrication of a photovoltaic device is now described by way of
reference to FIGS. 2-4. To begin the photovoltaic device
fabrication process, a substrate 202 is provided. See FIG. 2. As
highlighted above, suitable substrates include, but are not limited
to, a metal foil substrate, a glass substrate, a ceramic substrate,
aluminum foil coated with a (conductive) layer of molybdenum, a
polymer substrate, and any combination thereof. Further, as
described above, if the substrate material itself is not inherently
conducting then the substrate is preferably coated with a
conductive coating/layer. This situation is depicted in FIG. 2,
wherein the substrate 202 has been coated with a layer 204 of
conductive material. Suitable conductive materials for forming
layer 204 include, but are not limited to, molybdenum (Mo), which
may be coated on the substrate 202 using sputtering or
evaporation.
[0073] Next, as illustrated in FIG. 3, a chalcogenide film 302 is
formed on the substrate 202. In the particular example shown in
FIG. 3, the substrate 202 is coated with the conductive layer 204
and the chalcogenide film 302 is formed on the conductive layer
204. Chalcogenide layer 302 may be formed on the substrate 202
using the techniques described in conjunction with the description
of methodology 100 of FIG. 1, above. The chalcogenide film 302 will
serve as an absorber layer of the device.
[0074] An n-type semiconducting layer 402 is then formed on the
kesterite layer 302. According to an exemplary embodiment, the
n-type semiconducting layer 402 is formed from zinc sulfide (ZnS),
cadmium sulfide (CdS), indium sulfide (InS or In.sub.2S.sub.3),
oxides thereof and/or selenides thereof, which is deposited on the
kesterite layer 302 using for example vacuum evaporation, chemical
bath deposition, electrochemical deposition, atomic layer
deposition (ALD), and Successive Ionic Layer Adsorption And
Reaction (SILAR). Next, a top electrode 404 is formed on the n-type
semiconducting layer 402. As highlighted above, the substrate 202
(if inherently conducting) or the layer 204 of conductive material
serves as a bottom electrode of the device. Top electrode 404 is
formed from a transparent conductive material, such as doped zinc
oxide (ZnO), indium-tin-oxide (ITO), doped tin oxide or carbon
nanotubes. The process for forming an electrode from these
materials would be apparent to one of skill in the art and thus is
not described further herein.
[0075] According to the present teachings, the addition of the
above-described organic additive(s) (such as urea) is considered to
be primarily responsible for grain structures and film morphology,
however, additionally added metal species, such as Na species can
also further fine-tune the grain structures and film
morphology.
[0076] For example, FIGS. 5A-D show the impact of urea and Na on
the grains structures and film morphology. Specifically, FIGS. 5A-D
are scanning electron micrograph images. The image shown in FIG. 5A
is a top view of a sample metal chalcogenide film prepared from ink
containing no urea but about 15 wt % of ammonium sulfide as a
source of sulfur to assist CZTS crystallization. The image shown in
FIG. 5C is a cross-sectional view of the same film as in FIG. 5A.
The image shown in FIG. 5B is a top view of a sample metal
chalcogenide film prepared from ink containing 0.2M urea and 0.5
at. % NaF. FIG. 5D is a cross-sectional view of the same film as in
FIG. 5B.
[0077] Compared to the film prepared from an ink without urea and
Na (see FIGS. 5A and 5C), urea and Na greatly promoted the growth
of CZTS grains and fixed the surface cracks (see FIGS. 5B and 5D).
In order to further distinguish the effect of urea and Na, inks
containing only Na, only urea and both urea and Na as additive(s)
were used to develop CZTS thin film and photovoltaic devices. See
SEM images in FIGS. 6A-6F.
[0078] Specifically, FIG. 6A is a top view of a sample metal
chalcogenide film prepared from ink using only Na as additive. FIG.
6B is a cross-sectional view of the same film as in FIG. 6A. It is
clear from FIGS. 6A and 6B that the film was cracked and the grain
size of such film was small. FIG. 6C is a top view of a sample
metal chalcogenide film prepared from ink using only urea as
additive. FIG. 6D is a cross-sectional view of the same film as in
FIG. 6C. The surface of the film is much less cracked and the
grains are much larger than the film shown in FIGS. 6A and 6B. FIG.
6E is a top view of a sample metal chalcogenide film prepared from
ink using both urea and Na as additives. FIG. 6F is a
cross-sectional view of the same film as in FIG. 6E. Thus, when
both urea and Na were added as additives, the surface is even less
cracked and the grain structures are better than the film developed
from urea only ink.
[0079] As a result, the performance of the photovoltaic devices
developed from the above mentioned films are highly related to the
grain structures and film morphology. See FIGS. 7A-C. Specifically,
FIG. 7A is a graph illustrating electrical characteristics of a
metal chalcogenide film prepared using only Na as an additive, FIG.
7B is a graph illustrating electrical characteristics of a metal
chalcogenide film prepared using only urea as an additive, and FIG.
7C is a graph illustrating electrical characteristics of a metal
chalcogenide film prepared using both Na and urea as additives.
[0080] The device from ink using only Na as an additive showed
quite low efficiency of 2.5% (FIG. 7A), which may be due to the
small grains and cracked surface shown in FIGS. 6A and 6B. While
with improved grains and surface morphology, the device prepared
from ink using only urea as an additive showed significantly
improved efficiency of 4.8% (FIG. 7B). Furthermore an ink
containing both urea and Na yielded a device with conversion
efficiency of 6.2%, which reflects the relatively good grain
structures and film morphology (FIG. 7C). This demonstrates that
urea is the primary additive to promote the grain growth and film
morphology and the Na effect is secondary. Notwithstanding this,
the present techniques encompass situations wherein both urea and
Na are added to the film.
[0081] Advantageously, use of the present techniques has yielded
devices with energy conversion efficiencies of 8.1% or greater with
urea only inks. See examples below. For instance, FIGS. 8A and 8B
(described below) show SEM images of the film prepared from a
urea-only ink and FIG. 9 (described below) illustrates the
characteristics of a photovoltaic device based on this film.
[0082] The present techniques are further described by way of
reference to the following non-limiting examples
EXAMPLE 1
CZTS Device Absorber Layer Preparation Using Area as Additive
[0083] 1. The preparation of precursor ink for thin film
deposition: An aqueous ink was prepared by first dissolving 1.015 g
of copper(II) chloride (CuCl.sub.2, 99.99%, anhydrous from
Sigma-Aldrich), 0.600 g of zinc chloride (ZnCl.sub.2, 99.99%,
anhydrous, from Alfa Aesar) and 0.519 mL of tin (IV) chloride
(SnCl.sub.4, 99.995%, anhydrous from Sigma-Aldrich) into 15 mL of
de-ionized water. This solution was then slowly added into a
mixture of 5 mL ammonium sulfide (40-44% wt. in water, from Strem
chemicals Inc.) and 5 mL deionized water under vigorous stirring.
After the mixing was finished, another 5 mL of ammonium sulfide
(40-44% wt. in water, from Strem chemicals Inc.) and 5 mL of
deionized water were added into the mixture under stirring. The
mixture was then stirred for 10 minutes and subjected to ultrasound
for 60 minutes. Then the mixture was stirred for another 2 hours. A
brownish well-mixed slurry was formed. The solid part of the slurry
(a mixture of metal sulfides) can be isolated by centrifugation at
3,500 rpm/min for 15 minutes. The solid part was then redispered
into deionized water and again separated from the mixture using
centrifugation. The washing and centrifuge process was repeated
twice; Sometimes, 1-2 mL of ammonium sulfide (40-44% wt. in water,
from Strem chemicals Inc.) was used to help in the separation.
After washing, the solid part was redispersed into deionized water
by stirring to form a final volume of 24 mL of metal sulfide
slurry. This constitutes the formation of metal chalcogenides in
liquid medium (as per step 102 of methodology 100 (see description
of FIG. 1, above). Optionally, NaF can be added at a concentration
of from 0 at. % to 10 at. %, preferably from 0 at. % to 1 at. %.
This constitutes step 104 in FIG. 1 (optional M3 metal chalcogenide
or M3 salt).
[0084] The final ink for film deposition was prepared by mixing 6
mL of the cleaned metal sulfides slurry, 2 mL of 2M urea aqueous
solution (BioReagent from Sigma-Aldrich) and 1 mL of deionized
water under vigorous stirring (as per step 106 of methodology 100
(see description of FIG. 1, above)). The ink was dispersed using
ultrasound for 30 min and then stirred overnight before deposition.
The ink preparation was performed in a nitrogen filled
glovebox.
[0085] 2. Thin Film Development:
[0086] The ink was deposited on a 1.times.1 inch or 2.times.2 inch
(2-mm-thick) Mo-coated soda lime glass using spin coating in a
nitrogen-filled glovebox (as per step 108 of methodology 100 (see
description of FIG. 1, above). For a 2.times.2 inch substrate, 300
.mu.L of ink was spread on the substrate, followed by a
spin-coating recipe of 200 rpm 2 seconds, 800 rpm for 45 seconds
and 1,200 rpm for 3 seconds. The film was completely dried after
spin coating. Then the film was annealed at 350.degree. C. for 2
minutes, followed by cooling to room temperature. This procedure
was repeated 10 times in order to build sufficient film thickness.
After the final layer was deposited, the film was heated at
650.degree. C. for 15 minutes in the presence of 10 mg of S;
optionally, SnS can be also added during annealing, with the amount
of added SnS varying from 1 .mu.g to 1 g, preferably, from 10 .mu.g
to 100 mg (as per step 110 of methodology 100 (see description of
FIG. 1, above). Then the film was slowly cooled down to room
temperature.
[0087] The film morphology was investigated by scanning electron
microscopy (SEM). See FIGS. 8A and 8B. Specifically, FIGS. 8A and
8B are scanning electron micrograph images. The image shown in FIG.
8A is a cross-sectional view of a sample metal chalcogenide film
prepared according to Example 1. FIG. 8B is a top view of the
sample from FIG. 8A. The photovoltaic conversion efficiency (8.1%)
of the device developed from such film is shown in FIG. 9.
EXAMPLE 2
CZTSSe Device Absorber Layer Preparation Using Urea as Additive
[0088] The preparation of precursor ink for thin film deposition:
An aqueous ink was prepared by first dissolving 1.015 g of
copper(II) chloride (CuCl.sub.2, 99.99%, anhydrous from
Sigma-Aldrich), 0.667 g of zinc chloride (ZnCl.sub.2, 99.99%,
anhydrous, from Alfa Aesar) and 0.519 mL of tin (IV) chloride
(SnCl.sub.4, 99.995%, anhydrous from Sigma-Aldrich) into 15 mL of
deionized water. This solution was then slowly added into a mixture
of 5 mL ammonium sulfide (40-44% wt. in water, from Strem chemicals
Inc.) and 5 mL deionized water under vigorous stirring. After the
mixing was finished, another 5 mL of ammonium sulfide (40-44% wt.
in water, from Strem chemicals Inc.) and 5 mL of deionized water
were added into the mixture under stirring. The mixture was then
stirred for 10 min and subjected to ultrasound for 60 minutes. Then
the mixture was stirred for another 2 hours. A brownish well-mixed
slurry was formed. The solid part of the slurry (a mixture of metal
sulfides) can be isolated by centrifugation at 3,500 rpm/min for 15
minutes. The solid part was then redispered into deionized water
and again separated from the mixture using centrifugation. The
washing and centrifuge process was repeated twice; Sometimes, 1-2
mL of ammonium sulfide (40-44% wt. in water, from Strem chemicals
Inc.) was used to help in the separation. After washing, the solid
part was redispersed into deionized water by stirring to form a
final volume of 24 mL of metal sulfide slurry. Optionally, NaF can
also be added to the slurry at a concentration of from 0 at. % to
10 at. %, preferably from 0 at. % to 1 at. %.
[0089] The final ink for film deposition was prepared by mixing 7
mL of the cleaned metal sulfides slurry, 2 mL of 1M urea aqueous
solution (BioReagent from Sigma-Aldrich) under vigorous stirring.
The ink was dispersed using ultrasound for 30 minutes and then
stirred overnight before deposition. The ink preparation was
performed in a nitrogen-filled glovebox.
[0090] 2. Thin Film Development:
[0091] The ink was deposited on a 1.times.1 inch or 2.times.2 inch
(2-mm-thick) Mo-coated soda lime glass using spin coating in a
nitrogen-filled glovebox. For a 2.times.2 inch substrate, 300 .mu.L
of ink was spread on the substrate, followed by a spin-coating
recipe of 200 rpm for 2 seconds, 800 rpm for 45 seconds and 1,200
rpm for 3 seconds. The film was completely dried after spin
coating. Then the film was annealed at 350.degree. C. for 2
minutes, followed by cooling to room temperature. This procedure
was repeated 11 times in order to build sufficient film thickness.
After the final layer was deposited, the film was heated at
650.degree. C. for 20 minutes in the presence of 20 mg Se pellet
creating a CZTSSe film; optionally, SnSe can be also added during
annealing, with the amount of added SnSe varying from 1 .mu.g to 1
g, preferably, from 10 .mu.g to 100 mg. Then the film was slowly
cooled down to room temperature. For comparison, a pure sulfide
(CZTS) film was also prepared by heating the film at 650.degree. C.
for 20 minutes in the presence of 10 mg S flake; optionally, SnS
can be also added during annealing, with the amount of added SnS
varying from 1 .mu.g to 1 g, preferably, from 10 .mu.g to 100
mg.
[0092] The film morphology was investigated by scanning electron
microscopy (SEM). See FIGS. 10A and 10B. Specifically, FIGS. 10A
and 10B are scanning electron micrograph images. The image shown in
FIG. 10A is a top view of a sample CZTSSe film prepared according
to Example 2. The image shown in FIG. 10B is a cross-sectional view
of the film of FIG. 10A.
[0093] The powder X-ray diffraction patterns of CZTSSe and CZTS
film showed the kesterite phase of both materials. See FIG. 11. The
photovoltaic conversion efficiency of the CZTSSe and CZTS devices
developed from such film is shown in FIG. 12. Clearly evident in
the device results is the shift in open circuit voltage and short
circuit current, demonstrating the substitution of Se for S in the
absorber layer.
EXAMPLE 3
CZTS Device Absorber Layer Preparation Using Thiourea as
Additive
[0094] The preparation of precursor ink for thin film deposition:
An aqueous ink was prepared by first dissolving 1.015 g of
copper(II) chloride (CuCl.sub.2, 99.99%, anhydrous from Alfa
Aesar), 0.667 g of zinc chloride (ZnCl.sub.2, 99.99%, anhydrous,
from Alfa Aesar) and 0.591 mL of tin (IV) chloride (SnCl.sub.4,
99.995%, anhydrous from Sigma-Aldrich) into 15 mL of deionized
water. This solution was then slowly added into a mixture of 5 mL
ammonium sulfide (40-44% wt. in water, from Strem chemicals Inc.)
and 5 mL deionized water under vigorous stirring. After the mixing
was finished, another 5 mL of ammonium sulfide (40-44% wt. in
water, from Strem chemicals Inc.) and 5 mL of deionized water were
added into the mixture under stirring. Then the mixture was stirred
for 10 minutes and subjected to ultrasound for 60 minutes. The
mixture was stirred for another 2 hours. A brownish well-mixed
slurry was formed and continued to stir overnight. Then the solid
part of slurry (a mixture of metal sulfides) was separated by
centrifugation at 3,500 rpm/min for 15 minutes. The solid part was
redispered into deionized water and separated from the mixture
using centrifugation. The washing and centrifugation process was
repeated twice; Sometimes, 1-2 mL of ammonium sulfide (40-44% wt.
in water, from Strem chemicals Inc.) was used to help in the
separation process. After washing, the solid part was redispersed
into deionized water by stirring, forming a final volume of 24 mL
of metal sulfide slurry. Optionally, NaF can also be added at a
concentration of from 0 at. % to 10 at. %, preferably from 0 at. %
to 1 at. %.
[0095] The final ink for film deposition was prepared by mixing 4
mL of the cleaned metal sulfides slurry, 1 mL of 1M thiourea
aqueous solution under vigorous stirring. Sometimes, the ink was
dispersed with the help of ultrasound for 30 min. The ink
preparation was performed in a nitrogen-filled glovebox.
[0096] 2. Thin Film Development:
[0097] The ink was deposited on a 1.times.1 inch or 2.times.2 inch
(2-mm-thick) Mo-coated soda lime glass using spin coating in a
nitrogen-filled glovebox. For a 2.times.2 inch substrate, 300 .mu.L
of ink was spread on the substrate, followed by a spin-coating
recipe of 200 rpm for 2 seconds, 800 rpm for 45 seconds and 1,200
rpm for 3 seconds. The film was completely dried after
spin-coating. Then the film was annealed at 350.degree. C. for 2
minutes, followed by cooling to room temperature. This procedure
was repeated 11 times in order to build sufficient film thickness.
After the final layer was deposited, the film was heated at
650.degree. C. for 20 minutes in the presence of 10 mg of S;
optionally, SnS can be also added during annealing, with the amount
of added SnS varying from 1 .mu.g to 1 g, preferably, from 10 .mu.g
to 100 mg. Then the film was slowly cooled down to room
temperature.
[0098] The film morphology was investigated by scanning electron
microscopy (SEM). See FIGS. 13A and B. Specifically, FIGS. 13A and
B are scanning electron micrograph images. The image shown in FIG.
13A is a top view of a sample metal chalcogenide film prepared
according to Example 3. The image shown in FIG. 13B is a
cross-sectional view of the film of FIG. 13A. The photovoltaic
conversion efficiency of the device developed from such film is
shown in FIG. 14.
[0099] Although illustrative embodiments of the present invention
have been described herein, it is to be understood that the
invention is not limited to those precise embodiments, and that
various other changes and modifications may be made by one skilled
in the art without departing from the scope of the invention.
* * * * *